Spiral moisture equaliser and method of using same

ABSTRACT

The temperature and moisture content of particulate material, especially tobacco, are adjusted to desired levels by contacting the material with a flow of air having a temperature and relative humidity which would be in equilibrium with the particulate material at said desired temperature and moisture content. A suitable apparatus for carrying out the process comprises a spiral tray for supporting the particulate material, which tray can be oscillated with an upward and rotatory motion so that the particulate material progresses up the spiral to an outlet. Means are provided for circulating air through the spiral and for adjusting the temperature and relative humidity thereof.

[ 51 Apr. 29, 1975 United States Patent [1 1 Neville 3.527.643 9/l970Drexler et 34/12 SPIRAL MOISTURE EQUALISER AND METHOD OF USING SAME 75Inventor: Richard Ernest Gartside Neville, Camby Salisbury, EnglandAIIOIHF), Agent, or Firm-George W. Price; Charles 1. Worth [73]Assignee: AMF Incorporated, White Plains,

ABSTRACT The temperature an d moisture content of particulate [22]Filed: Feb. 27, 1974 rotatory motion so that the particulate materialpro gresses up the spiral to an outlet. Means are provided ReferencesCited UNITED STATES PATENTS for circulating air through the spiral andfor adjusting the temperature and relative humidity thereof.

18 Claims, 3 Drawing Figures m m m "n "on c k C wd o m LAF 77-5 746 899H 5-J-I- 097. U 34 90a 7-3 SPIRAL MOISTURE EQUALISER AND METHOD OF USINGSAME BACKGROUND TO THE INVENTION This invention concerns improvements tothe standard deviation of moisture content in tobacco or otherparticulate materials, but especially those with a large surface area tovolume ratio as for example, laminate materials such as cut tobaccoleaf.

There is a need for uniform moisture contents throughout tobacco leafprocessing. but particularly in the dried and cooled cut tobacco (rag)fed to the cigarette making machines. The need here is for every gram ofrag which goes to make a cigarette to have the same particular moisturecontent as the next.

There is a scatter in the moisture content of tobacco fed to thecigarette maker and the minimum cigarette weight and therefore, targetweight are aimed at giving acceptable firmness allowing for thisscatter. If the moisture content scatter can be reduced the cigaretteminimum weight and target weight can be reduced, again increasing theyield of cigarettes per pound of tobacco.

The most uniform moisture content body of tobacco that can be achievedis that in which all the pieces of tobacco are in equilibrium, i.e.,have the same temperature and the same vapour pressure and all thegrades are evenly blended throughout the mass. The moisture content ofeach piece of tobacco will not be the same. Grade, crop, position on theplant, curing etc., will all affect the vapour pressure for a particularmoisture content, so equilibrium is reached with all the pieces atvarying moisture contents. If the pieces are evenly mixed or blended anysample taken for moisture testing will consist of an equal mixture ofmoisture contents and hence given an equal moisture content reading. Soa uniform moisture content depends on blending as much as equilibriumtemperatures and vapour pressures.

Blending at present is achieved firstly by lamina blending bins whichcoarse blend the various grades throughout an operation by layering, sothat each 100 to 200 lbs discharged contains equal proportions of eachgrade. Secondly, by the stem add-back system which by measuring the flowof cut lamina on a weighing conveyor, to lb at a time, adds back thecorrect weight of cut stem. And thirdly, by a rotary drier which bytumbling about 20 lbs of rag per foot of cylinder aims at fine blendingthe cut strands, so that each cigarette weight contains the same blend.

Moderately accurate moisture contents are obtained at present bymeasuring the moisture content of tobacco before drying and aftercooling by electrical means and using the measurement to control theheat to the drier to obtain the desired output. But at the best, thismethod only corrects the long term variations or the mean moisturecontent of very large masses of tobacco compared with the weight in onecigarette.

Using the best apparatus available at present tobacco can be cooledafter drying to within 5 of ambient, but generally cooling is not asgood as that.

Final equalising of moisture and temperature are at present attempted bystoring the tobacco in storage silos. Whilst some equalising of adjacentpieces of to bacco occurs it is not possible in a reasonable time andusing a reasonable space for remote pieces to equalise since theeffectiveness of bulking is limited by the rate of diffusion of vapourin a relatively high bulk density mass of tobacco.

The object of the present invention is to provide a means for continuoushigh speed bulking to smooth the short term fluctuations in moisturecontent and to correct the moisture content and tobacco temperature topre-set values with high accuracy.

SUMMARY OF THE lNVENTlON According to the invention there is thereforeprovided a method for adjusting the temperature and moisture content ofparticulate material to desired levels, comprising passing a flow of airthrough the particulate material, the air having a temperature andrelative humidity which would be in equilibrium with the particulatematerial at said desired moisture content and temperature prior topassing through the particulate material.

The process employed for equalising or smoothing and correcting of, forexample, tobacco moisture content is adiabatic absorbtion or desorbtioncarried out at room temperatures. The heat of absorbtion or desorb tionis passed to, or provided by, the air. The process is a slow one becausethe only temperature difference which is available to transfer the heatinvolved is the modified wet bulb or reverse wet bulb temperature whichthe tobacco assumes. Modified because the water in the tobacco is notfree moisture so the vapour pressure is reduced, and reversed, i.e.,hotter than the air when the tobacco is absorbing moisture rather thandesorbing or evaporating. The rate of absorbtion or desorbtion isproportional to the difference between the vapour pressure of thetobacco and the air, i.e., proportional to the difference in moisturecontent of the tobacco from the equilibrium moisture content. So tobaccowhich deviates from the equilibrium moisture content adjustsexponentially towards the equilibrium, whilst tobacco which is at ornear the equilibrium is substantially unaffected.

Air of extremely accurate relative humidity is required. For rag tobaccoat 14% moisture content after drying and cooling a typical equilibriumcondition is 60% relative humidity at F. A change in relative humidityof 2 /2 to 5%, depending on the blend, is equivalent to l% tobaccomoisture content. So, relative humidity should be controlled to within i0.25%. A change in air temperature of 1F alters the relative humidity ofair with a fixed vapour content, by approximately 3.3%. so airtemperature must also be controlled very accurately to within i 0.08F.in order to generate a continuous supply of fresh air with a relativehumidity and temperature of a suitable accuracy, it is preferred tofully recirculate the air.

The thermal capacity of the air circulated is small compared with thethermal capacity of the tobacco on the conveyor, similarly the weight ofwater vapour in the air is small compared with the weight of water inthe tobacco. So the air leaving the tobacco reflects the meantemperature of the tobacco and assumes a relative humidity inequilibrium with the mean moisture content of the tobacco.

The temperature of the air returned to the tobacco may be controlledthermostatically, for example, by heating or cooling coils and thiscorrects the tobacco to the desired temperature. This temperaturecontrol need not be of very high accuracy as the relative humidity ofthe air is self-adjusting to the equilibrium condition by the removal oraddition of a negligible amount of moisture from the tobacco.

The moisture content of the tobacco is smoothed by the airrecirculation, but the mean is unaltered. To correct the mean moisturecontent, water vapour is added or removed from the recirculating air ata rate which is in accordance with the removal or addition required bythe tobacco.

The water vapour can be added evaporatively by me tered water spraywhich may be atomised by pressure, compressed air or spinning dischumidifier in which case the addition is accompanied by adiabaticcooling of the air. It can also be added as vapour using a metered steamflow through one or more nozzles or by heating a wick or water surfacewith a metered water and heat supply, in which case the air temperatureis substantially unaltered.

Water vapour can be removed by by-passing an adjustable proportion ofair through a bed of dessicant such as silica gel or throughrefrigerated coils to con dense the vapour.

The control of the amount of water vapour added or substracted from therecirculating air does not need to be of particularly high orderv Thecorrection to the tobacco would typically not exceed l% moisturecontent, so that a larger error of in the vapour substracted from oradded to the air would only cause a 0.1% moisture content error in thecorrection.

The control can be carried out in two ways. In the first, control isbased on the measurement of tobacco moisture content by a continuousmoisture meter. In its simplest form this would be a single measurementafter the moisture equalising conveyor, the moisture signal being fed toa feed back process controller where the signal is compared with thedesired moisture content and control signals generated to adjust thevapour addition or removal.

In a more comprehensive system additional measurements of moisturecontent and tobacco flow rate are made before the moisture equalisingconveyor and a water vapour addition or subtraction computed. Thecomputed signal is used as a feed-forward control to set the watervapour addition or substraction. The feedback control can be combinedwith the feed-forward to correct any errors resulting from thefeed-forward control.

The above method depends on the accuracy of continuous moisturemeasurement. There are various known electrical methods available, whichgive acceptable results, but none of which give the accuracy desired bythe industry.

In the second method of controlling the addition or removal of watervapour from the air and hence the mean moisture content of the tobacco,the relative humidity of the air is measured and held constant byadjusting the vapour removal or addition. For a well blended mixture oftobacco the equilibrium relative humidity is a measure of the tobaccomoisture content. The relative humidity of the air can be measured byvarious known methods and the measurement signal then compared with adesired value in a process controller which generates control signal toadjust the amount of vapour added or removed so as to maintain aconstant relative humidity.

In practise it is more convenient to slightly over-dry the tobacco inthe drying and cooling stage and only add moisture (and remove heat) inthe equaliser. There are various reasons for this:

I. Tobacco absorbs moisture more readily than it desorbs,

2. Cooling coils are required to remove the heat of absorbtion(Desorbtion would require heating coils) 3. Cooling coils are requiredin any case to remove the heat generated by the fan used to circulatethe air,

4. The tobacco is generally insufficiently cooled by existing coolers sothat further cooling is required in the equaliser. Cooling coils areagain required to remove this heat,

5. It is easier to add a measured weight of water with metering pump orflow meter, than it is to remove a measured weight.

6. There is a slight hysteresis in the absorbtion and desorbtion ofmoisture into leaf, so it is preferable that the equaliser either addsmoisture or removes it rather than both.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 shows a spiral tobacco cooler of known con struction in acut-away isometric view,

FIG. 2 shows a schematic sectional elevation of an example of thepresent invention is which the cooler of FIG. I has been modified with arecirculatory system, and

FIG. 3 is a plan view of the modified system.

DESCRIPTION OF PREFERRED EMBODIMENTS The arrangement shown in FIG. I isa spiral cooler in which the tobacco enters via a feed conveyor 10 ontoa spiral perforated tray 11 supported towards its centre by a cylinder13 forming a central core. The tray is provided with an upstanding rim14 at its outer edge and and is formed beneath with an air duct (plenumchamber) 16, which communicates by holes 17 with the interior of thecylinder 13.

The spiral tray conveys the tobacco upwardly by oscillation of the trayby means of a drive unit 19 (not shown in detail) which imparts anupward and rotational movement to the tray, the tobacco thereby beingthrown up the spiral of the tray, in a continuous series of steps untilit is discharged by a vibratory conveyor 20.

Cooling air enters through an inlet 22, being drawn in by an infusionfan 23 which drives the air down the cylinder 13, through the holes 17into the duct 16 and from there through the tobacco on the tray. The airthen passes over the rim and down the side of the tray and out throughthe exhaust air duct 25.

For carrying out the process of the invention, the tobacco conditioningarrangement shown in FIGS. 2 and 3 is provided in which a spiral coolersubstantially as shown in FIG. 1, suitably adapted. A feed conveyor 26leads to the lower part of a spiral perforated tray 28 supported towardsits centre by a cylinder 29 forming a central core. The tray is providedwith an upstanding rim 3] at its outer edge and is formed beneath withan air duct (plenum chamber) 32, which communicates by holes 34 with theinterior of the cylinder 29.

A drive unit 35 (not shown in detail) is provided for oscillating thespiral tray 28 by imparting an upward and rotational movement to thetray.

A spearate linear vibrating conveyor 37 leads away from the top of thespiral tray.

The spiral tray 28 is supported on links 38 and is enclosed in a heatinsulated stationary casing 40.

A circumferential slot 41 in the casing floor connects with ascroll-shaped or circular tapered duct 42 which leads to the inlet of anair conditioning unit 43 comprising an insulated casing 44 having at itslower end a high efficiency fan 46, connected by a divergent cowl 47 toa full flow roughing roll filter 49 situated above the fan. A furtherfilter 50 of the panel type is arranged between the roll filter and amulti-row finned tube air cooler 51, the cooling coils of which aresupplied with cooling water from a mains supply 52 via a modulatingvalve 54. The outlet 53 of the air conditioning unit is connected via aduct 55, having an air turn device 56, to the upper end of the cylinder29 supporting the tray 28 by way of a flexible connector 59. The duct 55contains water vapour dispensers in the form of steam nozzles 60,controlled from a relative humidity sensor 62 located within the duct 55close to the air inlet to the cylinder 29 and associated relativehumidity control means (not shown). The nozzles 60 are supplied withsteam from a supply pipe 61 via a modulating valve 65. The duct 55 alsocontains an air temperature sensor 64 and associated temperature controlmeans (not shown), which serves to control the water flow through thetubes of the cooler 51.

The equaliser shown in FlGS. 2 and 3 operates as follows:

Tobacco which has been dried to a moisture content below that desiredand cooled to a temperature above the desired temperature is fed fromconveyor 26 onto the spiral tray 28. Air is introduced into the spiralconveyor via the central core, whence it radiates to the duct (plenumchamber) below the perforated tray via the holes in the cylinder wall,which with a pressure drop across them assist in the air distribution.The air is withdrawn from the casing via the circumferential slot 41 tothe scroll-shaped or circular tapered duct 42. Air passes up through thetobacco on the tray whence heat from the tobacco is transferred to theair at a rate dependant upon the temperature difference therebetween andmoisture is absorbed from the air at a rate dependant upon the moisturecontent of the tobacco and the relative humidity of the air. Thus theair, after passing through the tobacco is hotter and drier thanpreviously.

The hot dry air then passes by way of the duct 42 to the airconditioning unit 43 where any dust carried in the air is removed byfilters 49 and 50. A measured amount of cold water is circulated throughthe coils of the cooler 51 to cool the air and a measured amount ofsteam is added to the air from the nozzles 60. As the air then returnsto the spiral tray by way of duct 55, it passes the temperature sensor64 and the relative humidity sensor 62. Temperature control meanscompare the sensed temperature with a predetermined temperature for theair and in accordance with the comparison adjust the modulating valve 54to alter the flow of cold water to the cooler 51. Relative humiditycontrol means compare the sensed relative humidity with a predeterminedrelative humidity and in accordance with the comparison adjust themodulating valve 65 to alter the flow of steam through the nozzles 60.Thus, the recirculating air enters the spiral tray at a controlledtemperature and relative humidity.

The temperature and relative humidity of the air entering the spiraltray are such as to be in equilibrium with the tobacco leaving thedevice at the desired temperature and moisture content.

The upward and rotational movement of the tray causes the tobacco to bethrown up the spiral of the tray in a continuous series of steps untilit is discharged by the conveyor 37.

The described system receives heat from three sources, which heat theair, namely, tobacco cooling, heat of absorbtion of moisture, and fancompression of The air conditioning unit is insulated and the equalisingconveyor is enclosed in an insulated casing 40 so heat interchange withthe ambient, if it differs from the controlled temperature, isnegligible. The heat of absorbtion is largely off-set by adiabaticcooling of the air if water sprays are used. The cooling coils aremultirow with a large heat transfer rate so that the coil temperaturewill not be below dew point, typically l0F below the air temperature.Dewing would remove moisture from the system, particularly where the airis stagnant in the coils around the headers.

The cooling coils are water-cooled, using mains water, which is eitherthrown away or recirculated through a cooling tower or refrigerator.

In addition to simplicity, the above recirculatory system has theadvantage of not requiring very accurate temperature control, and,because the system is selfadjusting for equilibrium relative humidity,not requiring very accurate control of water vapour addition.

The equalising conveyor can be fluid bed. gauze band conveyor orperforated tray vibrating conveyor. The main requirements are a longretention time of around 5 to l5 minutes and a large ventilating surfacearea to bring the maximum air in contact with each pound of tobacco.However, a perforated spiral tray is preferred since a fluid bed is notvery suitable for tobacco as the surface area is small and tobaccoproducts are not generally fluidisable. Gauze band conveyors may beused. although the air tends to bypass the deeper or denser parts of thetobacco layer if, as is inevitable, the tobacco carpet is not uniform.

The most suitable ventilating means is the vibrating tray conveyor withperforated bottom. A particular advantage is that the air resistance ofthe perforated tray bottom can be made several times greater than thedeepest part of the tobacco layer, by using a perforation which givesonly about 2%% free area, e.g., l/l6 inch diameter holes at inchstaggered pitch. By this means a uniform air flow can be obtained whichis substantially independent of the tobacco layer depth. This iseconomical in air flow and permits a smaller ventilating area than wouldbe required using a gauze band.

A band type ventilating means can also be constructed using a band withperforations having the same characteristic as for the vibrating tray.

In a particular form of construction, a spiral tray is used which givesa long length of tray in a short space, e.g., 10 turns of 6 feet-9inches OD X 4 feet-9 inches lD tray gives I feet-0 inches or 12 incheswide tray, which with a recirculating air flow of 8,000 c.f.m. and atobacco retention time of 5 to 10 minutes is suitable for equalising2,000 lb/hour of tobacco, and reducing the standard deviation ofmoisture content by a factor of approximately 3 x.

The air conditioning unit and connections to the spiral conveyor arecarefully sealed against leaks so that only the spiral conveyor casingis open to atmosphere at the tobacco inlet and outlet tunnels, which aresealed from draughts by one or more curtain seals. In this way, there issubstantially no interchange of the air in the system with ambient air.

It is also possible to provide sensors in the path of the air which isnot recirculated. the sensors controlling the temperature and humidityofa supply of air to the conditioning unit from a separate unit.

I claim: 1. A method of providing particulate material with thetemperature and moisture content thereof adjusted to desired levels,comprising the steps of providing a flow of particulate material andsimultaneously providing a recirculating flow of air,

adjusting the temperature and relative humidity of the flow of air tolevels to be in equilibrium with the temperature and humidity of theparticulate material when the temperature and humidity of such materialare at the desired levels,

passing the air flow with adjusted temperature and humidity through theparticulate material, and recirculating the flow of air that has passedthrough the particulate material.

2. The method of claim 1, wherein the temperature of the recirculatingair is controlled thermostatically.

3. The method of claim 1 wherein the relative humidity of therecirculating air is controlled by adding water vapour to therecirculating air.

4. The method of claim 1, wherein the relative humidity of therecircling air is controlled by removing water vapour from therecirculating air.

5. The method of claim 3, wherein the water vapour is addedevaporatively by a metered water spray.

6. The method of claim 3, wherein the water vapour is added as a meteredsteam flow.

7. The method of claim 4 in which the water vapour is removed by passingan adjustable portion of the recycling air through a dessicant orthrough refrigerated coils.

8. The method of claim 3 wherein the relative humidity of the air ismeasured before passing through the particulate material and is heldconstant by adjusting the rates of water vapour addition.

9. The method of claim 4 wherein the relative humidity of the air ismeasured before passing through the particulate material and is heldconstant by adjusting the rate of water vapour removal.

10. The method of claim 1, including the further steps of: supportingthe particulate material on a spiral tray; oscillating the tray with anupward and rotatory motion to cause the particulate material to progressupwards along the tray;

passing the air upwardly through the particulate material substantiallyover the length of the tray; cooling and adding water vapour to the airafter passing it through the particulate material; measuring thetemperature and relative humidity of the air;

maintaining the temperature and relative humidity constant by adjustmentof the cooling rate and the rate of addition of water vapour; andrecycling the air through the particulate material as it progressesalong the tray.

11. An apparatus comprising;

a. a spiral tray;

b. means for feeding particulate material to the spiral tray;

0. drive means for oscillating the spiral tray with an upward androtatory motion to cause particulate material to progress up the spiral;

d. means for removing particulate material from the top of the spiraltray;

e. means for recycling air through the spiral tray;

f. means for cooling and adding water vapour to the recycling air;

g. sensors positioned for measuring the temperature and relativehumidity of the recycling air; and.

h. control means for adjusting the cooling rate and the rate of additionof water vapour in accordance with any difference between the measuredtemperature and relative humidity and the temperature and relativehumidity of air which would be in equilibrium with the particulatematerial at said desired temperature and moisture content.

12. The apparatus of claim 11, wherein the means for removingparticulate material from the top of the spiral tray comprises a linearvibrating conveyor.

13. The apparatus of claim 11, wherein the means for recycling comprisesa fan, ducting for conveying air from the spiral tray to the fan andducting for conveying air from the fan to the spiral tray.

14. The apparatus of claim 11 wherein the means for cooling therecycling air comprises cooling coils in the path of the recycling airthrough which coils cooling fluid passes.

15. The apparatus of claim 14, wherein the control means for adjustingthe cooling rate comprises a modu lating valve in the supply of coolingfluid to the cooling coils.

16. The apparatus of claim It wherein the means for adding water vapourto the recycling air comprises at least one steam spray nozzle in thepath ofthe recycling air.

17. The apparatus of claim 16, wherein the control means for adjustingthe rate of addition of water vapour comprises a modulating valve in asteam supply to the at least one spray nozzle.

18. The apparatus of claim ll, further comprising a fiiter in the pathof the recycling air.

1. A method of providing particulate material with the temperature andmoisture content thereof adjusted to desired levels, comprising thesteps of providing a flow of particulate material and simultaneouslyproviding a recirculating flow of air, adjusting the temperature andrelative humidity of the flow of air to levels to be in equilibrium withthe temperature and humidity of the particulate material when thetemperature and humidity of such material are at the desired levels,passing the air flow with adjusted temperature and humidity through theparticulate material, and recirculating the flow of air that has passedthrough the particulate material.
 2. The method of claim 1, wherein thetemperature of the recirculating air is controlled thermostatically. 3.The method of claim 1 wherein the relative humidity of the recirculatingair is controlled by adding water vapour to the recirculating air. 4.The method of claim 1, wherein the relative humidity of the recirclingair is controlled by removing water vapour from the recirculating air.5. The method of claim 3, wherein the water vapour is addedevaporatively by a metered water spray.
 6. The method of claim 3,wherein the water vapour is added as a metered steam flow.
 7. The methodof claim 4 in which the water vapour is removed by passing an adjustableportion of the recycling air through a dessicant or through refrigeratedcoils.
 8. The method of claim 3 wherein the relative humidity of the airis measured before passing through the particulate material and is heldconstant by adjusting the rates of water vapour addition.
 9. The methodof claim 4 wherein the relative humidity of the air is measured beforepassing through the particulate material and is held constant byadjusting the rate of water vapour removal.
 10. The method of claim 1,including the further steps of: supporting the particulate material on aspiral tray; osciLlating the tray with an upward and rotatory motion tocause the particulate material to progress upwards along the tray;passing the air upwardly through the particulate material substantiallyover the length of the tray; cooling and adding water vapour to the airafter passing it through the particulate material; measuring thetemperature and relative humidity of the air; maintaining thetemperature and relative humidity constant by adjustment of the coolingrate and the rate of addition of water vapour; and recycling the airthrough the particulate material as it progresses along the tray.
 11. Anapparatus comprising; a. a spiral tray; b. means for feeding particulatematerial to the spiral tray; c. drive means for oscillating the spiraltray with an upward and rotatory motion to cause particulate material toprogress up the spiral; d. means for removing particulate material fromthe top of the spiral tray; e. means for recycling air through thespiral tray; f. means for cooling and adding water vapour to therecycling air; g. sensors positioned for measuring the temperature andrelative humidity of the recycling air; and, h. control means foradjusting the cooling rate and the rate of addition of water vapour inaccordance with any difference between the measured temperature andrelative humidity and the temperature and relative humidity of air whichwould be in equilibrium with the particulate material at said desiredtemperature and moisture content.
 12. The apparatus of claim 11, whereinthe means for removing particulate material from the top of the spiraltray comprises a linear vibrating conveyor.
 13. The apparatus of claim11, wherein the means for recycling comprises a fan, ducting forconveying air from the spiral tray to the fan and ducting for conveyingair from the fan to the spiral tray.
 14. The apparatus of claim 11wherein the means for cooling the recycling air comprises cooling coilsin the path of the recycling air through which coils cooling fluidpasses.
 15. The apparatus of claim 14, wherein the control means foradjusting the cooling rate comprises a modulating valve in the supply ofcooling fluid to the cooling coils.
 16. The apparatus of claim 11wherein the means for adding water vapour to the recycling air comprisesat least one steam spray nozzle in the path of the recycling air. 17.The apparatus of claim 16, wherein the control means for adjusting therate of addition of water vapour comprises a modulating valve in a steamsupply to the at least one spray nozzle.
 18. The apparatus of claim 11,further comprising a filter in the path of the recycling air.